NHS hospitals need to be prepared for hackers attempting to hold their computer systems to ransom, a doctor has argued in a leading medical journal.

He warned it was likely that an increasing number of hospitals would be shut down by ransomware attacks, where hackers gain access to a computer system and will only release control in return for payment.

The article, published in The BMJ, cited estimates that almost a third of NHS trusts had been infected by ransomware.

Dr Krishna Chinthapalli, a neurology registrar at the National Hospital for Neurology and Neurosurgery in London, wrote: "We should be prepared: more hospitals will almost certainly be shut down by ransomware this year."

He argued that hospitals were "ideal targets" for ransomware companies as they had irreplaceable data and may be "more willing" than other organisations to pay for quick recovery of their data.

Data held by hospitals, including patients' birth dates and addresses, could be sold on, he added.

The article stated that nine in 10 NHS trusts ran an "obsolete" version of Windows.